Loay Hussein -- a political prisoner from 1984 to 1991 -- was taken from his home in the Sehnaya district near the Syrian capital of Damascus, according to the country's Observatory for Human Rights.

Hussein had been supporting protesters who marched for a sixth straight day, chanting, "The people want to bring down the regime," a spokesman to the organizers told CNN from the southern city of Daraa.

The organizers are planning a day of mass protests Friday, he added.

Tuesday's protesters paraded through the Syrian towns of Sanamain and Jassem.

On Monday, protesters marched in Daraa after they buried the body of a protester who was killed Sunday in clashes between anti-government demonstrators and security forces, an eyewitness told CNN.

Sunday's protests came the same day a delegation from President Bashar al-Assad offered "condolences to the families of the two martyrs who died during the unfortunate events which took place in Daraa on Friday," according to SANA, the Syrian news agency.

Local tribal leaders have said they rejected a government proposal to bring about calm and have put together a list of demands including asking that the head of the local police and governor of Daraa be held accountable for the civilian casualties since Friday, witnesses said.

The United Nations Human Rights office reported that six people have been killed by security forces in the southern city of Daraa since Friday, calling for an "independent, transparent and effective investigation into these killings."

"The use of excessive force constitutes a clear violation of international law, which provides for individual criminal responsibility for violations committed," said Rupert Colville, spokesman for the U.N. Office of the High Commissioner for Human Rights.

"People have the legitimate right to express their grievances and demands to their government, and we urge the Syrian government to enter into a broad, meaningful dialogue with the protesters in an attempt to address those grievances," he said.

Opponents of the al-Assad government allege massive human rights abuses and are calling for political and economic reforms.

Syria is the latest in a string of Arabic-speaking nations beset with discontent.